This invention relates to the construction of electromagnetic transmission line elements including resonating, coupling and wave-guiding elements, and more particularly, to the construction of such elements by use of a boundary between two dielectric materials of high and low dielectric constants, the low dielectric constant being in the range of approximately 1-2 and the high dielectric constant being in the range of 80-100 or higher.
One well known form of transmission line structure employs a region of metallic material separated from a second region of metallic material by a region of electrically insulating material. Such a transmission line structure includes microstrip wherein an electrically conductive strip is separated from a parallel conducting plate by a layer of insulating material. As a further example of transmission line, a coplanar waveguide comprises a pair of parallel conductive strips spaced apart by an insulator. The latter structure, in combination with an insulated back metallic plate or ground plane as in stripline or microstrip, can also serve as a coupler of microwave signals between two microstrip circuits, upon a reduction in the spacing between the conductive strips. In similar fashion, two or more electrically insulated conductive strips, patches or resonators may be disposed in a coplanar array spaced apart from a ground plane to serve as a filter, or may be stacked, one above the other and insulated from each other to form a filter. In the latter configuration of stacked resonators, it is the practice to enclose, at least partially, each of the resonators in a metallic cavity type of structure with provision for electromagnetic coupling between the resonators.
In each of the foregoing structures, the physical size of the structure, for provision of a desired electromagnetic characteristic, is determined by the electromagnetic wavelength in air, vacuum, or dielectric environment in which the metallic elements are situate. However, there are situations such as in communication via satellite, wherein it is desirable to reduce the physical size and weight of the microwave components and the circuitry composed of such components. Microwave components of unduly large size and weight create a packaging problem for satellite borne electronic equipment.
The foregoing problem may be demonstrated by the following example concerning microwave filters. Filters of electromagnetic signals, such as microwave signals, typically provide a bandpass function characterized by a multiple-pole transmission band. A typical construction employs a plurality of metallic resonators of planar form which are stacked one above the other to provide for plural modes of electromagnetic vibration within a single filter. The resonators are spaced apart and supported by dielectric, electrically-insulating material. Metallic plates with irises may be disposed between the resonators for coupling electromagnetic power among the resonators. In the case of cavity-resonator filters, each cavity is physically large, particularly at lower frequencies, the physical size militating against the use of the cavity filters. Thus, in situations wherein there is limited space available for electronic circuits, such as in satellites which serve as part of a communication system, there is a need to reduce the size of filters, as well as to decrease the weight of filters employed in the signal processing circuitry.
The filters are employed in numerous circuits for signal processing, communication, and other functions. Of particular interest herein are circuits, such as those which may be constructed on a printed circuit board, and are operable at microwave frequencies, such as frequencies in the gigahertz region. Such signals may be processed by transistors and other solid state devices, and may employ analog filters in the form of a series of cavity resonators, or resonators configured in microstrip form. By way of example, to provide a band-pass filter having an elliptic function or a Chebyshev response, and wherein a mathematical representation of the response is characterized by numerous poles, the filter has many sections. Each section has a single resonator, in the microstrip form of circuit, for each pole which is to be produced in the filter transfer function.
In order to reduce the physical size of such a filter, the filter may be constructed of a series of dielectric resonators enclosed within metallic cavities, as is disclosed in Fiedziuszko, U.S. Pat. No. 4,489,293, this patent describing the construction and tuning of a multiple, dielectric-loaded, cavity filter. Such a dielectric resonator filter is employed in situations requiring reduced physical size and weight of the filter, as is desirable in a satellite communication system wherein such a filter is to be carried on board the satellite as a part of microwave circuitry. The reduction in size of such a filter arises because the wavelength of an electromagnetic signal within a dielectric resonator is substantially smaller than the wavelength of the same electromagnetic signal in vacuum or in air. Coupling of electromagnetic power between contiguous cavities may be accomplished by means of slotted irises or other electromagnetic coupling structures.
The foregoing attempts to reduce the size of microwave components, such as the foregoing filters, by use of dielectric materials have been successful to a limited extent, the limitation devolving from the fact that, in the case of the foregoing filters, the inner space of a cavity is filled partially with air and partially with the dielectric resonator. Furthermore, as noted above for satellite communications, it is important also to reduce the weight of the microwave components, and such weight reduction is limited in the foregoing construction of filter due to the fact that the cavity walls and iris plates are constructed of metal rather than than a lighter material. Thus, there is a need to treat further the foregoing problem of excess size and weight.